Because of the complicated effects involved in making Xerxes a 10 foot giant, Rodrigo Santoro filmed almost all of his scenes alone in a green-screen room and had little/no interaction with the rest of the cast. He said it was difficult but he got used to it as he had also done it on the first film.
Eva Green was the only actress considered for the role of Artemisia and luckily for the film-makers she agreed to do it as they had no second choice.
Sullivan Stapleton thought Gerard Butler looked "really huge" in the original 300 (2006) and that he would similarly have to bulk up and get a lot bigger as he was playing the lead role this time around. But he was actually asked to slim down because he is playing an Athenian and not a Spartan, he was at 10-12 kilos below his usual weight. All The Athenians in the film are also shown as having "six pack abs" like the Spartans but the Athenians are not as big.
Sullivan Stapleton was surprised by how wordy the film was. He thought he would "just be roaring and chopping off limbs throughout" but actually has a great number of dialog scenes along with fight scenes.